Calibrating method for artifact-reduced MRT imaging when there is FOV displacement

Abstract

The present invention relates generally to nuclear magnetic resonance tomography (synonym: magnetic resonance tomography—MRT) as used in medical applications for examining patients; it relates in particular to a calibrating method when there is FOV displacement within a plane through angle-dependent modulating of the phase of the Fourier DC point of the respective read-out direction in MRT imaging for determining the device-specific dependency of the time delay requiring to be compensated between the reference signal effecting modulation and the read-out measuring data points of the respective read-out direction of settable measurement parameters.

1. A calibrating method for determining a device-specific time delay dt between a reference signal and a measuring data point in a read-out direction when having a FOV displacement within a plane in MRT imaging, comprising:
 specifying the read-out direction; 
 measuring a plurality of phase values in a Fourier DC point for different FOV displacements in the read-out direction; 
 determining the device-specific time delay as a slope of a linear function between the FOV displacements and the phase values; and 
 repeating the measuring and determining steps for a variation of a measurement parameter until a mathematical correlation is determined between the device-specific time delay and the measurement parameter. 
 
   
   
     2. The calibrating method as claimed in  claim 1 , wherein the measurement parameter is selected from the group consisting: size of the FOV, resolution, bandwidth, sequence type, scanner type, layer orientation, and scanning rate t of radial frequency space scanning. 
   
   
     3. The calibrating method as claimed in  claim 2 , wherein the measurement parameter is the scanning rate t of radial frequency space scanning which has the largest impact on the time delay. 
   
   
     4. The calibrating method as claimed in  claim 1 , wherein the mathematical correlation is dt=A·t+B, where t is indirectly proportional to a product of bandwidth and resolution. 
   
   
     5. The calibrating method as claimed in  claim 1 , wherein measuring the phase values in the Fourier DC point is based on averaging complex values of Fourier-transformed measuring points of the read-out direction. 
   
   
     6. The calibrating method as claimed in  claim 1 , wherein the Fourier-transformed measuring points in the read-out direction are weighted with their magnitude square during averaging. 
   
   
     7. The calibrating method as claimed in  claim 1 , wherein the measuring is performed on a homogeneously signal-transmitting phantom contained within the FOV. 
   
   
     8. A measuring method for artifact-reduced MRT imaging when having a FOV displacement, comprising:
 generating high-frequency exciting pulses; 
 radially scanning a signal in a frequency space by gradient pulses using an analog-to-digital converter; 
 superimposing a reference signal on the radially scanned signal of a read-out direction,
 wherein a frequency of the reference signal is dependent on the read-out direction, 
 wherein a phase of the reference signal relative to the radially scanned signal is calibrated so that a phase in a Fourier DC point of the read-out direction is independent of the frequency of the reference signal; and 
 
 performing a Fourier transformation on the radially scanned signal to obtain an artifact-reduced image in a local space. 
 
   
   
     9. A correction method for artifact-reduced MRT imaging when having a FOV displacement, comprising:
 generating high-frequency exciting pulses; 
 radially scanned signal in a frequency space by gradient pulses using an analog-to-digital converter; 
 superimposing a reference signal on the radially scanned signal of a read-out direction,
 wherein a frequency of the reference signal is dependent on the read-out direction; 
 
 determining a read-out direction specific error phase dφ i  due to a time delay between the radially scanned signal and the reference signal; 
 correcting a phase of the radially scanned signal by multiplying a phase-correction factor exp(−idφ i ) in the read-out direction; and 
 performing a Fourier transformation on the phase-corrected radially scanned signal to obtain an artifact-reduced image in a local space.